42 F. B. SUMXER AND R. R. liUESTIS. 



2. The R :G ratio is slightly, though perhaps significantly, higher 

 in fraterculus than in cremicus. In other words, the darker race 

 seems to be relatively redder, the paler race relatively yellower. 

 It was earlier observed that a dark race (ntbidits) and a pale race 

 (sonoriensis) of another species (maniculatus} differed in this 

 same direction, though to a greater extent. 



3. The mean values for the l generation of hybrids lie, in each 

 case, between the values for the two parent races, though they are 

 not in any case midway in position. 



4. The mean values for the F., generation differ somewhat from 

 the Fj values, but they are none the less intermediate between 

 those of the parent races with the exception of the figure for the 

 R :G ratio. 



5. The standard deviations for the F. 2 generation exceed those 

 for the F! in the case of black, white and color, the differences be- 

 ing 5.4, 3.8 and 3.6 times their probable errors, respectively. 

 There is no such increase of variability, however, in the case of 

 the R :G ratio, the standard deviations for which are approxi- 

 mately equal in the two generations. It will be noted that the 

 three former " characters " differ rather widely in the two parent 

 races, whereas the difference in the latter case is slight. 



The statistical certainty of the color differences between the 

 subspecies which were crossed, and the fact that these differences 

 persist, apparently undiminished, after several generations in cap- 

 tivity, show that they are largely genetic in their nature. The 

 same is not so evident with respect to the individual differences 

 within a single race or within a group of hybrids of similar origin. 

 Observation shows that part of these differences are due to age or 

 to condition of molt ; while season is perhaps likewise responsible to 

 a certain extent. Previous studies * have shown, however, that a 

 certain fraction of this individual variability has a genetic basis. 

 For the present material, coefficients have been computed, indi- 

 cating the degree of parent-offspring correlation with respect to 

 the four values employed in our computations (black, white, color 

 and the R:G ratio). The resulting figures are given in the fol- 

 lowing table. 



1 Sumner, Journ. E.vp. Zool., Oct., 1923. 



